Summary

This is an in class activity designed to help students discover WHY P and S waves behave the way they do and how they are affected by their environment. It also goes into how we use seismic waves to determine the geologic structure of the earth (on a very basic level). Key words to describe this activity are; seismic, P-wave, S-wave, interior of the earth, and geophysics.

Context

Audience

I have used this activity in every class I teach where there is the slightest excuse to discuss earthquakes. I think it is critical to for student understanding of how we know things about earthquakes at a distance and how we can use that information to teach us about the interior of the earth. I have yet to have a class that doesn't appreciate this activity from about to graduate seniors to first-years.

Skills and concepts that students must have mastered

Before beginning the activity students must know what an earthquake is and that it generates P and S-waves. More information is helpful, but this activity can lead you into a discussion of seismograms or the interior of the earth or occur about half-way through. I would not plan on using it at the end. I use it when they begin to look glazed.

How the activity is situated in the course

As above, this activity usually occurs during the introduction of earthquakes and/or the interior of the earth. It should occur either at the beginning of that section or 1/3 to ½ way through depending on how you teach it.

Goals

Content/concepts goals for this activity

Students using this activity will understand why P-waves travel faster than S-waves, P-waves travel through fluids and S-waves don't, and waves travel faster through denser materials.

Students will be able to determine the vague shape of a fluid filled body in a solid using P and S-waves

Higher order thinking skills goals for this activity

Hypothesis testing
Discovery analysis

Other skills goals for this activity

Description and Teaching Materials

The Wave Race: A Seismic Wave Discovery Activity: By Dr. Erin Beutel
Question 1: Which goes faster, the P-wave or the S-wave and why?
Step 1: Divide the class into 2 equal length lines, if your class is not equally divided pull one person out to be the pusher for the other line (the lines must be the same length and the same number of students). The lines should face each other (the students in one line should be able to see the students in the other line (like they were about to begin a dance).
Step 2: Give the last student in each line a stop watch the instructions to start it when you say "Go" and stop it when the 'wave' gets to them.
Step 3: Explain loudly and repeatedly (and they will still do it) that they cannot move until they feel the person next to them MOVE them.
Step 4: Have them link arms and space themselves evenly. Go to each line and demonstrate the two waves that will be racing, the P-wave and the S-wave.

The P-wave is created by gently pushing against the shoulder of the first person in line, they then bump the shoulder of the next person and so on.

The S-wave is created by pulling the first person in line forward (bend them forward at the waist), because their arms are linked they will pull the following person forward as well.

Step 5: (This works best with LONG lines of people, at least 6 in each line, 12 + is better). Set up someone at the head of the P-wave to give it a gentle push when you say go. Position yourself at the head of the S-wave. When you say go, the stop watches should start and both waves should start. Watch for people moving ahead of the wave (and remember their arms are linked).
Outcome 1: The S-wave should be slower than the P-wave. The explanation being that, like in the lines, the S-wave has to move particles further which takes more time. To demonstrate that it isn't just that line, switch the P and S lines and do the experiment again.

Question 2: How does density affect the rate of wave travel?
Step 1: Have one line space themselves further apart and one line move closer together. Use the same wave for each line (if you need to you can also switch people around to make the waves the same length) and once again time how long it takes for each wave to travel the length of the line (P-waves work best). Watch carefully for 'advance' movement in the spaced line, each student must FEEL the wave before they move, not just see it. (Remember because the lines are facing each other they can see the effect the waves are having on the other line and how they are moving).
Outcome 2: The more dense line should be faster.
Step 2: If you have enough people you can create lines of varying density and talk about how the wave changes speed and then talk about refraction vs reflection. You can even measure the density change and the rate change and do calculations.

Question 3: How do fluids affect seismic wave travel?
Step 1: With your original 2 lines, space them evenly apart and then have them unlink arms.
Step 2: Do the same experiment as is Question 1, have one line do the S-wave and one line do the P-wave.
Outcome 3: By unlinking their arms, the students are more representative of a fluid, the P-wave will travel through the fluid, the S-wave will not.

Question 4: How do geophysicists use this information to determine information about the interior of the Earth?
This is a harder step and is best done in a simplified manner, though you can get as complicated and realistic as you like.

Step 1: In a large open space outline s spheres inside each other to represent the Lithosphere/Mantle and the Outer Core (this experiment does not deal with how we know the interior of the earth is a solid).
Step 2: Ask the students (after having done the previous 3 experiments) to determine where they would want the seismic lines to go through to determine as much as possible about the interior. Depending on how much time you have you can direct them as much or as little as possible. The outcome you want is that they come up with the idea that the earth has a liquid area and approximately how large it is.
Step 3: On a white board/chalkboard etc draw a single and the paths that they will have the seismic waves travel. Then have them do P and S-wave lines along each of these paths and time them (if you want to create denser areas closer to center timing matters, if you are simplifying it then the time doesn't matter, just which waves made it and which didn't).
Step 4: Have the students map which paths had both waves go through and which did not and then use this information to determine something about the interior of the circle. See how close they came to the circle you used to represent the outer core. Talk about whether you can get all the information you need out of one earthquake or if more is better and test.
Example:

Outcome 4a: Students are able to see how the interior of the Earth was imaged and also to begin to understand how much data was necessary. This can lead into a competition where you have the shape of a fluid body in a solid 'planet' in mind. Each team has a certain number of earthquakes/ray paths they can pick. You draw the circle on the board and draw each line they want and say whether the S-wave went through or not. The team with the closest shape/location of your fluid body wins.

Outcome 4b: They should be able to map the approximate shape of the liquid outer core and understand in general terms how we know what the interior of the Earth is like.

Assessment
This activity can be designed as a lab write-up where given the questions students must come up with a hypothesis and then based on the test in class write-up their results. This is a great way for them to solidify what they have learned since so many of them think they get it when you do the experiment, but then forget or get confused later.

Earthquake Dance: How do seismic waves move and cause damage?
This is a stand along activity or can be combined with the seismic wave races. Students should form 2 lines facing each other, or if you are in a super-smart classroom, a camera that projects them onto the screen for all to see.

Overview: Students will use their bodies to imitate (in general terms) the motion or particles when hit by one of the seismic waves.
Step 1: The P-wave: Have students step and back.
Step 2: The S-wave: Have students bend their knees to go up and down
Step 3: The Love-Wave: Have students step side-to-side.
Step 4: The Rayleigh-Wave: Have students do a full wave motion with their entire bodies (step forward with knees bent, rise, step back and bend knees again). Repeat.
Step 5: Do all 4 waves at once.
Outcome: Students remember the general motion of the various seismic waves and a discussion about what the most damaging scenario's might be and how the waves cause damage ensues. This can be combined with a seismic building activity where students build structures to resist earthquakes.

Teaching Notes and Tips

Assessment

This activity can be designed as a lab write-up where given the questions students must come up with a hypothesis and then based on the test in class write-up their results. This is a great way for them to solidify what they have learned since so many of them think they get it when you do the experiment, but then forget or get confused later.

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